EuMg\(_2\) Bi\(_2\)  and EuMg\(_2\) Sb\(_2\)  crystals showed puzzling features in the field-dependent \(ab\) -plane magnetic susceptibility \(\chi _{ab}(T)\)  below \(T_{\rm N}\) . For EuMg\(_2\) Bi\(_2\) , The \(\chi (T<T_{\rm N})\)  measurements for \(H\parallel ab\)  and \(H\parallel c\)  in a field of 1 kOe were the same (isotropic) [1]}, whereas for A-type AFM order one instead expects from molecular-field theory (MFT) that \(\chi _{ab}(T\rightarrow 0)=\chi (T_{\rm N})/2\)  and \(\chi _c(T\rightarrow 0)=\chi (T_{\rm N})\)  [2]}, [3]}, [4]}. The authors of Ref. [1]} suggested that this observed behavior arises from \(c\) -axis magnetic ordering instead of the above A-type \(ab\) -plane ordering subsequently reported [6]}. Similarly, the data for \(\chi _{ab}(T\le T_{\rm N})\)  in Ref. [7]} for \(H=0.5\) –30 kOe were nearly independent of \(T\) . These authors suggested that this \(T\) -independent \(\chi _{ab}(T\le T_{\rm N})\)  behavior might arise from a field-induced helical magnetic structure with a turn angle of \(\approx 120^\circ \)  based on MFT [2]}, which we now know is not the correct magnetic structure [6]}. A similar anomalous behavior of \(\chi _{ab}(H,T)\)  was recently observed for EuMg\(_2\) Sb\(_2\)  crystals [10]}.
